A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticule, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An important step in a typical lithographic process is aligning the substrate to the lithographic apparatus so that the image of the mask pattern is projected at the correct position on the substrate. Semiconductor, and other, devices manufactured by lithographic techniques may require multiple exposures to form multiple layers in the device, and it is may be essential that these layers line up correctly. As ever smaller features are imaged, overlay requirements, and hence the accuracy required of the alignment process, become stricter.
In one known alignment system, described in EP-A-0,906,590 which document is hereby incorporated by reference, marks on the substrate comprise two pairs of reference gratings, one X and one Y, with the two gratings of the pair having slightly different periods. The gratings are illuminated with spatially coherent light and the diffracted light is collected and imaged on a detector array, the different diffraction orders having been separated so that corresponding positive and negative orders interfere. Each detector in the array comprises a reference grating and a photo detector. As the substrate is scanned, the output of the detector varies sinusoidally. When the signals from both gratings of a pair peak simultaneously, the mark is aligned. This type of system provides a large dynamic range and by using high diffraction orders, is relatively insensitive to mark asymmetry. However, the need to provide two gratings with different periods increases the amount of space required for the alignment marks on the substrate. It is desirable to minimize the amount of such “silicon real estate”devoted to alignment marks and therefore not available for production of devices, or for other purposes.
Another known alignment system, described in EP-A-1,148,390 which document is hereby incorporated by reference, uses a compact self-referencing interferometer to generate two overlapping images rotated over +90° and −90° which are then made to interfere in a pupil plane. An optical system and (optional) spatial filter selects and separates the first order beams and re-images them on a detector. The system described in EP-A-1,148,390 utilizes a special technique, also described as self-referencing on center of symmetry of an alignment mark. Also, this alignment system uses the envelope of the detected signal to determine the correct alignment position.